Various particle sizing samplers are available on the market today for sampling particles according to size which are bourne by a gaseous medium as air. These samplers are generally concerned with those size particles entrained in a gaseous medium which are harzardous to humans because they are breathable. Because sampling devices are concerned only with breathable size particles, their sampling size range is generally for particles about ten microns or less in diameter. The volumeric flow rates through any particluar one of these particle sizing samplers may vary depending on the particular application, usually from less than one cfm to more than forty cfm.
One of the problems with these air samplers is that, many times, the gaseous medium being sampled contains some particles which are significantly larger in diameter than the particle size acceptance range of the samplers. When the gaseous medium being sampled contains these larger particles, the larger particles bounce off the impaction surface and reentrain themselves in the gas stream. These reentrained particles are subsequently separated out on the lower stages and thereby skew the size distribution data so as to make the particle size distribution and mass medium diameter appear smaller than it actually is.
Attemps have been made to preliminarily separate those particles from the gas stream just prior to passage through the particle sizing sampler which are significantly larger than the sampler particle size acceptance range such as the use of a small cyclone preseparator. Because the diameter of a cyclone at any given flow rate determines the size of the particles collected thereby, the required diameter of a cyclone used to preseparate only those particles with diameters significantly larger than those in the sampler particle size acceptance range frequently resulted in the required diameter of a preseparator cyclone being significantly larger than the diameter of the particle sizing sampler. In some instances, this has been a problem because the diameter of the sampler access openings through the ducts carrying the gases to be sampled is about the same diameter as the particle sizing samplers, thereby making it difficult to use a preseparator cyclone with the particle sizing sampler since the preseparator cyclone diameter may be larger than these access openings in the ducts through which the preseparator cyclone and the particle sizing sampler must be inserted to accurately sample the particles according to size. The only way to reduce the preseparator cyclone diameter is to reduce the flow rate through the sampler and this is frequently at undesirable alternative.
Another problem with preseparator cyclones is that there are no satisfactory theoretical formulations to predict the behavior of the cyclone at various gas temperatures and pressures. This makes it necessary to calibrate the cyclone empirically at every gas temperature and pressure combination which might be encountered in actual use. This calibration procedure is expensive to perform and makes cyclones less desirable as sampling instruments than impactors.
Another problem encountered when using prior art preseparators with particle sizing samplers is that the particles which were preseparated from the gas stream by the preseparator were sometimes undesirably reentrained in the gas stream and carried into the particle sizing sampler. This is especially a problem when sampling is being attempted under conditions where high grain loadings of the larger size particles in the gas stream being sampled were present. Another problem encountered in prior art preseparators is that the collection capacity of the preseparator was not sufficient to permit the preseparator and thus the particle sizing sampler to remain in the gase stream for the necessary length of time required for adequate sampling without distortion of the particle preseparation characteristics of the preseparator.